Waste papers of the aforementioned types often contain aluminum in amounts of approximately 5 to 10 kg/ton of bone-dry pulp. This aluminum has usually been added in the form of alum as a retention agent during the prior papermaking process for these paper products. It is known that the presence of this aluminum creates problems of incrustation in evaporation plants. It is also known that magnesium compounds can be added thereto for the purpose of reducing these problems. Aluminum can thus be precipitated in the form of aluminum-magnesium compounds, and in this manner the amount of dissolved aluminum in the pulp suspension can be decreased.
In view of the relatively high aluminum content in these waste papers, however, such measures are generally not sufficient to make it possible to recycle white water from the delignification step to the evaporation plant. The presence of too great an amount of dissolved aluminum being returned to the evaporation step would in such a case give rise to the aforementioned incrustation problems.
It is desirable and necessary, however, to recycle the white water from the delignification step, particularly in view of the chemical and COD-content of the white water.